Project summary: Axons are typically viewed as an all-or-none transmission line between the neuronal soma and presynaptic terminals. To test this assumption, we circumvented some of the limitations of classic techniques for studying unmyelinated fibers. We have refined recently developed extracellular single-axon recording techniques. We record extracellularlyfrom single fibers while simultaneously recording intracellularly from the soma of origin. We can perform these recordings under normal or seizure-promoting conditions. Preliminary experiments support the feasibility of performing axonal recording up to 800 uM away from the soma of origin in area CAS of hippocampus. Action potentials can be stimulated either orthodromically (intracellularly) from the natural site of initiation or can be stimulated antidromically from the focal extracellular recording electrode. We propose to use this technique to address fundamental unanswered questions regarding the basis for observations of axonal plasticity in small CMS unmyelinated fibers. We provide preliminary evidence that axons possess more dynamic signaling flexibility than has been assumed. We suggest that depression of axonal signaling during seizure events may actually be an endogenous mechanism for depressing seizure propagation. This mechanism could be exploited and amplified by future anti-convulsant strategies once the mechanisms are better understood. We also present evidence for considerable timing plasticity attributable to both initiation site plasticity and conduction velocity changes. We will use CAS region of hippocampus as an initial model system because of the well studied anatomical, physiological, and synaptic properties of these cells, and also because of the relevance to previous work on seizure mechanisms in this area. However, a great advantage of the technique is that it can be applied to many cell types. Therefore, we will explore modulation of axonal behavior in other cells of the hippocampus to address unresolved questions regarding axonal behavior and to test the overarching hypothesis that axons possess more computational power than previously assumed. Relevance: The research in this proposal will aid the understanding of the etiology and propagation of seizures. Our results will also lead to cellular insights into the mechanism of brain stimulation techniques currently used in the treatment of movement disorders and neuropsychiatric disease.